Traditional voice calls can be made via a Public Switched Telephone Network (PSTN). This refers to one or a combination of the world's circuit switched telephone networks, over which users' telephones can addressed based on the standardized system of telephone numbers. PSTN networks include landline networks as well as mobile cellular networks. Thus a mobile phone can make and receive phone calls over the PSTN network, by means of a dialler application installed on the mobile phone which uses a wireless transceiver in the phone to connect to a base station of the cellular network via the voice channel. The dialler application can then connect to another mobile or static terminal over the cellular network (and if not on the same network, onwards via another cellular network or landline network of the PSTN).
It is also known to enable a user to conduct a voice call over a packet-switched network, which may comprise a single proprietary network or a combination of multiple constituent networks. E.g. the packet-switched network may comprise a private intranet such as a company intranet, and/or a wide area internetwork such as that commonly referred to as the Internet. In the case of calls made using Internet Protocol as the packet protocol, such calls are referred to as Voice over Internet Protocol (VoIP)) calls. To be able to make or receive a packet-switched call such as a VoIP call, the user at each end has a communication client application installed on his or her respective user terminal (or equivalently uses his or her user terminal to access a server-hosted instance of the client, e.g. a web-hosted instance accessed via a general purpose web browser on the user terminal). The client application uses a network interface on the user terminal to connect to the packet-switched network (e.g. Internet), and thereby establish a communication session with the other user terminal over the packet-switched network.
For packet-switched calls, the user terminal can be a traditional desktop computer, but can also be a mobile terminal such as a smartphone, tablet or laptop, or even a wearable mobile terminal such as a smart watch or smart glasses. The mobile terminal may connect to the packet-switched network (e.g. the Internet) via the data channel of the cellular network, or via a wireless access point of a wireless local area network (WLAN), e.g. via a short-range radio frequency (RF) technology such as Wi-Fi or Bluetooth.
It is also known for a hybrid call to be conducted part way over a PSTN network and part way over a packet-switched network such as the Internet. To achieve this, a gateway is provided between the two different types of network. Thus the user terminal can connect to the gateway by PSTN and the user terminal on the other end of the call can connect to the gateway via the circuit-switched network, or vice versa, with the call being routed via the gateway.
Presently the calling user (the user who initiates the call) chooses manually prior to the call whether to make a packet-switched or PSTN call, and the call remains conducted via the same type of network throughout. In one known system, there is provided a PSTN fallback whereby if the calling user sends a call establishment request to the callee user over the Internet requesting a call by VoIP, but the called user does not answer, then the client application on the calling user's terminal automatically attempts to redial the callee via a PSTN network.